ZocDoc CEO: We’ll be nationwide by end of year

FORTUNE — Startup ZocDoc has announced plans to rapidly expand from servicing only 40% of the U.S. — or nearly 2,000 cities and towns — to 100% by the end of this year.

“We know with medical appointments, people want to be booking their appointments online,” says ZocDoc CEO Cyrus Massoumi. “It’s not just a big-city phenomenon — it’s something we know will work in even the most rural parts of the United States.”

After a bad experience booking a doctor’s appointment, Massoumi wanted to help patients more easily locate doctors and book appointments with them online. So in 2007, he founded ZocDoc with over $90 million in funding from backers including Jeff Bezos, Marc Benioff, Khosla Ventures, and SV Angel.

With ZocDoc, users have a free solution for locating a practitioner online and booking an appointment to see them within one to three days, significantly less than the national average wait time of 18.5 days. (ZocDoc accomplishes this largely through tapping into last-minute openings made available by other patients’ cancellations.)

ZocDoc appears to be gaining steam. The company, which employs 500, has seen rapid growth over the last year alone, doubling its user base to 5 million in the last six months. According to Massoumi, one of ZocDoc’s priorities moving forward will be offering new features like its check-in feature, introduced in late 2012, which lets users fill out their medical forms online ahead of time. Offers Massoumi: “There are so many ways to improve that white space between doctor and patient.”